Pop-up's are sometimes empty...

I am working with different IBM tools, API Connect, DataPower, WAS, etc. and I have noticed that some "popup" windows (there's a ton of popup's in IBM's tools for various settings) are opening up as empty windows. The common factor is that they are always "modal" with "locked" address bar.
I have not seen this issue in any other browsers (not even IE or Edge).

Most of these popup's is triggered from JavaScript, for example: onclick="doActionNew( 'TCPConnectionTest', '', '', 'false')"

If I right-click and do a "Reload" I get a URL/Address in the bar but also a HTTP 500 as the opening token is then missing from the server:

As the server token is missing after Reload/Refresh of the pop-up Window I hit the "TCP Connection Test" button again without closing the HTTP 500 window in between and then I get the "real" page in the pop-up:

Why is this...?
I assume a Vivaldi bug as I can't replicate the same behavior in any other browser...

There are no errors or related information in the Console.

This is very annoying and I really love Vivaldi so please help me fix!